1. Field of the Invention
This invention relates broadly to water-based coating compositions and in particular to polymer binder emulsions for use in water-based coating compositions. More particularly, this invention relates to polymer binder emulsions for use in formulating protective coatings, primer, primer surfacer, and sealer compositions for metallic substrates.
2. Description of the Prior Art
Acrylic polymer lacquers, i.e., a liquid coating composition consisting of film-forming polymers such as alkyl acrylates and alkyl methacrylates dissolved in a solvent, are well known in the art and have been extensively utilized as exterior coatings for automobiles and trucks. In particular, lacquers of methyl methacrylate as shown in Johnson, U.S. Pat. Nos. 2,783,131 and 2,782,132, both issued Feb. 19, 1957; Crissiy, U.S. Pat. No. 2,934,509, issued Apr. 26, 1960 and Parker, U.S. Pat. No. 3,477,969, issued Nov. 11, 1969, have been utilized for that purpose. However, such lacquers in general lack the necessary adhesion properties and corrosion and rust resistance to enable them to be applied directly to the base metal substrate. It is therefore necessary to apply a primer composition to the metal substrate to achieve the necessary corrosion and rust resistance properties and to provide a surface to which the acrylic lacquer coatings will adhere. Moreover, where the primer composition is based on alkyd resins to which acrylic lacquers do not adhere well, an intermediate coat or sealer is commonly applied over the primer to provide a surface to which the acrylic lacquers will firmly adhere.
Primer and sealer compositions based upon epoxy-acrylic polymeric binders are known in the art. Thus, Rohrbacher, U.S. Pat. No. 3,509,086, issued Apr. 28, 1970, discloses a sealer composition comprising a polymeric binder of an acrylic polymer and a high molecular weight epoxy resin dissolved in an organic solvent and Walus, U.S. Pat. No. 3,707,516 discloses a coating composition useful as a primer, sealerless primer, or sealer comprising an acrylic-epoxy graft copolymer dissolved in a solvent. The epoxy-acrylic compositions disclosed in the aforementioned patents have a number of properties which make them attractive as primer and sealer compositions. For example, the compositions disclosed therein exhibit excellent chip and mar resistance, good adhesion to metal and improved corrosion resistance properties. However, these prior art sealer and primer compositions possess several serious disadvantages. In particular, these prior art compositions have the serious disadvantage of being solvent based compositions. Thus, these solvent-based compositions possess the serious disadvantage of emitting solvents to the atmosphere when they are dried thereby contributing to environmental pollution problems. Moreover, the solvents employed in such compositions are generally quite expensive and solvent reclamation is therefore desirable. However, present solvent reclamation techniques are quite complex and expensive. In view of the increased emphasis on environmental pollution control and the recently proposed legislation directed to the elimination of photochemically reactive solvents, these are serious disadvantages indeed.
It is accordingly a principal object of the present invention to provide water-based coating compositions for use as primer and sealer compositions to overcome the problem of solvent emissions. It is a further, more specific object of the present invention to provide water-based epoxy-acrylic compositions which retain most of the advantages of the prior art epoxy acrylic solvent-based compositions without their disadvantages.